Annually, intrauterine infection with cytomegalovirus (CMV) causes between 1,000 and 3,000 infants to be mentally retarded and/or deaf. These effects occur following primary maternal CMV infection during the first half of pregnancy. Seronegative mothers with children under age two years attending daycare and the seronegative women caring for these children have significantly increased risk of acquiring CMV from their children or children in their care. The rate of infection in these women is between 10 percent and 30 percent per year compared to a rate of <five percent among other women. The majority of women are planning additional children and this may have increased the incidence of primary maternal infection during pregnancy. annual infection rates of 30 percent make a vaccine trial feasible and the live attenuated Towne vaccine will be tested in seronegative women with young children shedding CMV. One hundred fifty women will be divided into vaccinees and placebo controls. The study will be randomized and double blinded. Another group of 300 naturally seropositive women will be monitored to determine the immunity afforded by wild type infection. Both groups will be matched for age, race, marital status, children in the home and socioeconomic status. Each woman and her family members will be monitored for three years. The prevalent strains of CMV shed by the children attending each daycare center will be determined. Vaccine efficacy will be assessed by comparing the incidence of CMV acquisition in vaccinees and controls and associated with the persistence of neutralizing antibodies, EIA antibodies, cell-mediated immunity, duration between vaccination and exposure, duration between vaccination and acquisition of CMv by the children of the vaccinees, and by strain genotype. Children born to vaccinees will be monitored for CMV infections for two years. This vaccine trial will determine if immunization against reinfection with an exogenous strain of CMV is possible.